Single phase undirectional surface acoustic wave transducer and improved reflectors

ABSTRACT

A unidirectional transducer for a surface acoustic wave (SAW) device. In one embodiment the device includes (1) a defined area on a piezoelectric substrate within which is located an open circuit reflector perpendicular to the SAW direction of propagation; and a pair of low reflectivity transducer electrodes located within the defined area and connected to opposing bus bars, the electrodes perpendicular to the direction of the SAW propagation and positioned with the excitation center of the pair of electrodes located about seven-eighths of a Rayleigh wavelength at a center frequency of the SAW from the reflector.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims the benefit of U.S. Provisional Application Ser.No. 60/509,693 filed on Oct. 8, 2003, entitled “Single PhaseUnidirectional Saw Transducer,” commonly assigned with the presentinvention and incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general to surface acoustic wave(SAW) devices and more specifically, to a single-phase unidirectionalsurface acoustic wave transducer and to improved reflectors for SAWdevices.

BACKGROUND OF THE INVENTION

The never ending trend toward smaller electronic devices increases thepressure to develop smaller and more efficient components. For example,wireless communications systems are requiring increasingly enhancedperformance from passive components used for signal processing,particularly those operating above one GHz. In the case of SAW filters,characteristics typically demanded include low insertion loss, lowpassband ripple, high degree of linearity of phase and high selectivity.To meet these demands single-phase unidirectional transducers (SPUDTs)are frequently used. SPUDT devices can also be used for SAW sensors andSAW radio frequency identification tags.

A SPUDT structure calls for the placement of reflectors and transducersin such a way that, within each unit cell, the center of transduction isshifted with respect to the center of reflection. Ideally, this phaseshift should be equal to ±one-half of pi (±π/2). In most SPUDTstructures, electrodes one-eighth of a Rayleigh SAW wavelength wide andreflectors ranging from one-fourth to three-eighths of a wavelength wideare used to obtain a nonreflecting transduction. In the majority ofcases the electrodes are one-eighth of a wavelength or narrower.Consequently, in the GHz range the critical dimensions of electrodes arebeyond the limits of feasibility for large scale fabrication techniquesbased on optical lithography.

For SAW devices operating at 2 GHz and higher frequencies, thewavelength is about 2 μm. Thus, an electrode one-eighth of a wavelengthwide has an absolute width of about 0.25 μm. With the thickness rangingfrom 2% to 10% of a wavelength, the absolute height of the electrode isabout 40–200 nm. This small aluminum cross section for the electrodecauses resistive losses to become unacceptably high. For this reason,SPUDT transducers are seldom used above 1 Ghz.

Accordingly, what is needed in the art is a low-loss unidirectionaltransducer that can operate on a substrate at frequencies higher than 1GHz that can be manufactured utilizing large scale fabricationtechniques based on optical lithography.

Also needed in the art are better reflector configurations to use withSAW radio frequency identification tags. In the case of SAWidentification tags, it is important that as much of the energyreflected in response to a transducer generated interrogation pulse becaptured as possible. If an aluminum reflector located on a substrate isthe same size as the transducer and if that reflector is straight andsubstantially perpendicular to the interrogation pulse, a substantialamount of energy generated by the transducer is not going to be includedin the reflected pulse. This is because a portion of the pulse generatedby the transducer does not impact a reflector due to the fact that itexpands in size as it travels down the SAW tag surface away from thetransducer.

Thus, what is needed in the art is a better reflector for use on a SAWtags that have the capability of capturing more of the interrogationpulse energy in order to return a more vigorous reflected signal to thetransducer.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides for a unidirectional transducer for a SAWdevice. In one embodiment the device includes (1) a defined area on apiezoelectric substrate within which is located an open circuitreflector perpendicular to the SAW direction of propagation; and (2) apair of low reflectivity transducer electrodes located within thedefined area and connected to opposing bus bars, the electrodesperpendicular to the direction of the SAW propagation and positionedwith the excitation center of the pair of electrodes located aboutseven-eighths of a Rayleigh wavelength at a center frequency of the SAWfrom the reflector.

Thus, the present invention provides for a transducer that willconcentrate the energy of the SAW in one direction on a SAW substrate.Such a device is advantageous in the case of such SAW devices as SAWidentification tags. For SAW RFID tags, it is desirable to deal withonly one set of reflected signals. In addition, the present inventionprovides for a more vigorous interrogation pulse because the energygenerated by the transducer that would have been otherwise discarded isadded to the energy of the SAW interrogation pulse.

In one embodiment the invention provides for the defined area to be adistance about equal to twice the Rayleigh wavelength at a centerfrequency of the SAW, plus the Rayleigh wavelength of a center frequencyof the SAW multiplied by a positive integer minus one. In anotherembodiment the transducer electrodes are separated by a distance ofabout one-half of the Rayleigh wavelength at a center frequency of theSAW. In still another embodiment the transducer electrodes each have awidth of about one-fourth of the Rayleigh wavelength at a centerfrequency of the SAW.

For manufacturing convenience it is advantageous to use aluminum forboth the reflector and the transducer electrodes. An embodiment of theinvention provides for the electrodes to have a low total reflectivity.In one of the embodiments of the invention the electrodes have athickness about equal to one-tenth of the Rayleigh wavelength at acenter frequency of the SAW. An advantageous embodiment of the inventionprovides for the device to have a piezoelectric substrate such that themechanical reflectivity has an opposite sign as compared to theelectrical part of the reflectivity of the pair of electrodes and thereflector. Such a piezoelectric substrate is 128° LiNbO3.

In still another embodiment the width of the electrodes is less thanone-quarter wavelength. In yet still another embodiment the width of thereflector is between 0.3 wavelength and 0.5 wavelength.

An advantageous embodiment of the invention provides for at least twoopen circuit reflectors having a width of about one-fourth wavelengthand separated by a distance of about one-half wavelength. In anotherembodiment of the invention the excitation center of the pair ofelectrodes, located about seven-eighths of a Rayleigh wavelength at acenter frequency of the SAW from the reflector, is varied by an amountequal to plus or minus ten percent of the seven-eighths of a Rayleighwavelength.

A useful embodiment of the invention provides for a plurality of pairsof electrodes with each of the pairs offset at a distance equal to anumber of wavelengths and connected to the same bus bars in the samesequence of polarity. In another embodiment the device is furthercomprised of a plurality of equivalent reflectors, with each reflectoroffset at a distance equal to a number of wavelengths from each othersuch that the reflectors do not overlap the electrodes.

Another useful embodiment of the device provides for it to be furthercomprised of a periodic set of defined areas. In this embodiment theperiodic set of the defined areas are situated quasi-periodically withthe period equal to the length of the defined area or greater than thelength by an integer number of wavelengths, the wavelength slowlychanging (chirped) along the length of the piezoelectric substrate. Theinvention as described herein is usefully employed as a unidirectionalSAW transducer for low loss applications.

Yet another embodiment of the invention provides for a plurality ofdefined areas placed in parallel acoustic sub channels, separated by awavelength, perpendicular to the propagation direction of the wave, andelectrically connected in parallel.

Of course a particularly useful application of the invention is for usein SAW identification tags. Thus, the device described herein isusefully employed when the defined space is located on a SAWidentification tag.

An extremely useful embodiment of the invention provides for a SAWdevice that is comprised of (1) a piezoelectric substrate with a SAWtransducer located thereon; and (2) a reflector on the substrate forreflecting a response to an interrogation pulse generated by the SAWtransducer, the reflector arranged on the substrate to substantiallymatch the diffraction field of the interrogation pulse. In oneembodiment the reflector is located in the far field of theinterrogation pulse while in another it is located in the near field.

Of course, the device may further be comprised of a plurality ofreflectors arranged on the substrate. In such case, at least one of theplurality of reflectors may be located in the near field and at leastone of the plurality of reflectors located in the far field of theinterrogation pulse and still be within the intended scope of thepresent invention.

The invention also provides for the reflector to be curved tosubstantially match a contour of constant phase in the diffraction fieldof the interrogation pulse. In still another embodiment, the reflectoris curved to substantially match a contour of constant amplitude in thediffraction field of the interrogation pulse. In yet still anotherembodiment the reflector is curved to substantially match both thecontour of constant phase and the contour of constant amplitude in thediffraction field of the interrogation pulse. A particularly usefulembodiment provides for the reflector to be segmented to form asubstantial curvature shape and substantially match either the contourof constant phase or the contour of constant amplitude in thediffraction field of the interrogation pulse.

The present invention can also be usefully employed to cause thereflector to focus a reflected pulse at the transducer such that thereflected signal substantially matches the amplitude and phasedistribution of the interrogation pulse.

The invention can be usefully employed when the reflector is an eitheran open circuit metallic strip or a short circuit. It can also beusefully employed when in the case of non-metallic reflectors. A usefulembodiment of the invention provides for the reflector to be segmented.In the case of a segmented reflector, a useful embodiment provides for aspace between each of the segments that is about equal to one-quarter ofa wavelength. In another embodiment the reflector is designed toencompass the main lobe and first sidelobes of the interrogation pulse.In an embodiment where the reflector encompasses the main lobe and firstsidelobes of the interrogation pulse, eight segments are used for themain lobe and four segments are used for each of the sidelobes for atotal of sixteen segments.

A significant embodiment of the invention provides for a SAW device thathas (1) a piezoelectric substrate with a SAW transducer located thereon;and (2) a reflector on the substrate for reflecting a response to aninterrogation pulse generated by the SAW transducer, where the reflectoris arranged on the substrate in a substantially curvature configurationto substantially match the amplitude and phase of the diffraction fieldof the interrogation pulse.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a unidirectional transducer device constructed inaccordance with the present invention;

FIG. 2 illustrates a variant of a unidirectional transducer constructedin accordance with the present invention utilizing more than one pair ofelectrodes;

FIG. 3 illustrates the adjustment of the transduction and reflectivityof a SAW device by utilizing a plurality of pairs of transducerelectrodes and reflectors;

FIG. 4 illustrates a basic unidirectional SAW transducer constructed inaccordance with the present invention utilizing periodically distributedSPUDT cells of the type illustrated in FIG. 1;

FIG. 5 illustrates a SAW filter utilizing at least one unidirectionaltransducer constructed in accordance with the present invention;

FIG. 6 illustrates parallel-connected SAW transducers generating surfaceacoustic waves in a common acoustic channel;

FIG. 7 illustrates a SAW tag utilizing at least one unidirectionaltransducer constructed in accordance with the present invention;

FIGS. 8A–8F illustrate diffracted wave forms for amplitude and phase ofan interrogation pulse on a piezoelectric substrate at various distancesfrom a transducer;

FIG. 9 illustrates a reflector on a piezoelectric constructed inaccordance with the present invention that is arranged to substantiallymatch the diffraction field of the interrogation pulse;

FIG. 10 is a representative SAW tag using reflectors of the typeillustrated in detail in FIG. 9; and

FIG. 11 illustrates a representative example of a segmented metallicopen-circuit reflector constructed in accordance with the presentinvention that substantially matches both the amplitude and a contour ofconstant phase of the diffraction field of an interrogation pulse.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a unidirectionaltransducer device 100 constructed in accordance with the presentinvention. Shown is a top view of a piezoelectric substrate 150 that hascell 110 located on its surface 155. The cell 110 has a length 120 ofabout twice the SAW Rayleigh wavelength (determined at the centerfrequency of the SAW that is generated on the piezoelectric substrate150). Shown in the cell 110 is a strongly reflecting one-half wavelengthwide floating or open circuit reflector 130 and a weakly reflecting pairof one-quarter wavelength wide electrodes 140 used to excite SAWs on thepiezoelectric substrate. Each of the electrodes 140 is connected to abus bar 160 such that the electrodes 140 are connected to bus bars 160of opposite polarity. The illustrated device 100 with a pair ofelectrodes 140 connected to bus bars 160 of opposite polarity wherebythe internal reflection of the floating reflector 130 is used to causethe electrodes 140 to be a unidirectional SAW transducer.

This invention provides for a unidirectional SAW transducer constructedon a piezoelectric substrate 150 in a space that has a length 120 ofabout twice the Rayleigh wavelength at the center frequency of the SAWplus the Rayleigh wavelength multiplied by a positive integer minus one.Within the embodiment illustrated in FIG. 1, the positive integer isassumed to be one, thus the length 120 is equal to twice the Rayleighwavelength. The direction of propagation 170 of the SAW is from left toright. The electrodes 140, which can be aluminum, are located aboutperpendicular to the direction of propagation 170 and are separated by adistance of about one-half a Rayleigh wavelength. Each of the electrodes140 is about one-fourth a Rayleigh wavelength wide and each is,respectively, connected to a bus bar 160 of opposite polarity.

Located within the cell 110, is the open circuited reflector 130 (whichcan be aluminum) oriented parallel to the pair of electrodes 140. Thereflector 130 is positioned so that the center of reflection is locatedat a distance of about seven-eighths of a wavelength from the excitationcenter of the pair of electrodes 140, which is taken to be the center ofthe gap between the electrodes 140. The predominant direction of the SAWbeing from the reflector 130 towards the pair of electrodes 140.

In some piezoelectric substrates 150 the mechanical part of reflectivityhas an opposite sign compared to that of the electrical part ofreflectivity. The use of such piezoelectric substrates 150 achieves lowlosses for one-quarter wavelength wide floating or short-circuitedreflectors 130 and electrodes 140 because of the substantialcancellation of the electrodes 140 total reflectivity. An example ofsuch a substrate material is 128° LiNbO₃ where one-quarter wavelengthwide electrodes 140 are weakly reflecting for aluminum electrode 140thicknesses that equal about 0.1% to 10% of a wavelength. Aluminum basedalloys, such as AlCu with a low percentage of Cu, and others can also beused as the material for the reflectors and electrodes. To achievemaximum reflectivity, the width of the open-circuited reflector 130 isbetween within 0.3 of a wavelength and 0.5 of a wavelength. The centerof reflection is at the approximate center of the wide short circuit orfloating reflector 130 and the center of excitation is at the center ofthe gap between the one-quarter wavelength wide electrodes 140. Thenominal distance from the reflection center to the excitation center canbe varied within ±10% of a wavelength by adjusting the position of thereflector 130 within the cell 110 by a corresponding shift to findoptimal unidirectionality. In FIG. 1, the SAW is predominantlypropagated to the right. A comparison of the generated and reflectedforward propagating waves (ignoring the reflectivity of the one-quarterwavelength wide fingers in the electrodes 140) within the cell 110 showsthat the phase difference equals 4π, and thus the propagated wavesinterfere constructively. The phase of the reflection coefficient is+π/2, with reference point at the center of an open reflector on 128°LiNbO3. For the opposite direction, the phase difference between thegenerated and the reflected signals is 5π, and they tend to cancel eachother. All critical dimensions in this structure, including gaps, are onthe order of one-quarter of a wavelength.

Where 128° LiNbO3 is used with one-quarter wavelength wide electrodes140, the electrodes 140 are weakly reflecting for metal thicknesses 0.1to 10% of a wavelength. In practice, for 128° LiNbO3_(i) aluminumthicknesses ranging from 1% to 8% are most suitable for the purposes ofthis invention. The thickness range is limited by increased resistivityfor low thicknesses and by difficulties in producing high-aspect-ratioaluminum profiles for high thicknesses. Moreover, for each particularthickness, a metallization ratio (a/p) corresponding to vanishingreflectivity of the electrodes 140 can be found or determined.

For example, in a short-circuited long grating, the reflectioncoefficient for a/p=0.5 is close to zero for the relative aluminumthickness of 2.5%. For higher thicknesses, the metallization ratio hasto be decreased to attain low reflectivity. For a single one-halfwavelength wide floating or open circuit reflector 130, the reflectioncoefficient is found to be significantly higher than that for shortcircuited one-quarter wavelength wide electrodes 140, the maximumoccurring for metallization ratios a/p of between 0.3 and 0.5.

It is evident that similar approaches can be applied to other substratesand materials for electrodes and reflectors exhibiting properties ofopposite sign mechanical and electrical reflectivity. Another examplethat satisfies this criterion is YZ-LiNbO3. A significant feature of theinvention is the use of the above-described structures as weaklyreflecting electrodes 140 and strong reflectors 130.

Turning now to FIG. 2, illustrated is an embodiment of a unidirectionaltransducer device 200 utilizing a transduce with a pair of reflectors230. In this embodiment, the cell 210 has a pair of floating or opencircuit reflectors 230 with each reflector 230 in the pair having awidth approximating one-quarter wavelength. There is a separationbetween each of the reflectors 230 in the pair of about one-halfwavelength. The center of reflection is taken to be in the middle of theseparation between the two reflectors 230. The nominal distance from thereflection center to the excitation center of the electrodes 240 isseven-eighths of a wavelength and can be varied within ±10% of a periodof the transducer electrodes 240 by adjusting the position of thereflectors 230 within the unit cell 210 by a corresponding shift to findthe optimal unidirectionality. The pair of reflectors 230 is placedbetween two pairs of transducer electrodes 240 which are sequentiallyconnected to two bus bars 260. The critical dimension of this structureis on the order of one-eighth of a wavelength, which renders thisstructure less attractive for high-frequency applications than thestructure described above with respect to FIG. 1.

Turning now to FIG. 3, illustrated is an embodiment where the SAW device300 includes at least one more pair of electrodes 345 that areessentially equivalent to the first pair of electrodes 340 with each ofthe additional electrode 345 pairs offset at a distance of a number ofwavelengths from the first pair of electrodes and connected to the samebus bars 360 in the same sequence of polarity as the first pair ofelectrodes 340.

In another embodiment, the SAW device includes at least one morereflector 345 essentially equivalent to the first reflector 340, each ofthe additional reflectors 335 offset at a distance of a number ofwavelengths from the first reflector 330, and, if the additional pairsof electrodes 345 are present, the number of wavelengths (m) will bem≠n+1 to avoid overlapping the reflectors 330, 335 with the transducerelectrodes 340, 345. Here, positive n, m correspond to a shift towardsthe right. This procedure allows one to vary the number of transducerpairs of electrodes 340, 345 and reflectors 330, 335 along thestructure, thus creating weighted unidirectional structures withimproved performances.

Turning now to FIG. 4, illustrated is a SAW device 400 that includes aperiodic set of identical sections described in FIG. 1, whereintransducer electrodes 440 are connected periodically to bus bars 460 ofopposite polarity, with a period equal to two wavelengths. Here, in eachsection, only one pair of electrodes 440 is used and the reflector 430only comprises one one-half wide wavelength and is floating oropen-circuited. This embodiment corresponds to an unweightedunidirectional SAW transducer. It is evident that in a single-endedcircuitry, the signal is connected to one of the bus bars 460 and theother busbar 460 is grounded. This unidirectional transducer can be usedfor generation of SAWs with low losses and has numerous applications assuch, e.g., for SAW sensors, actuators, et cetera.

Turning now to FIG. 5, illustrated is an embodiment of the invention forfilter applications, wherein at least one unidirectional SAW transducer500 is connected to input bus bars 560, operating as an input transducerand generating SAWs in some acoustical channel 570, and a receivingtransducer 590, illustrated schematically, connected to the output busbars 565 placed in the same acoustic channel 570. It should be notedthat the receiving transducer 590 has more than two electrodes 545. Itshould also be noted that a transducer may have any number of electrodesin any of the embodiments of the present invention and still be withinthe intended scope of the invention.

Turning now to FIG. 6, illustrated is an embodiment of the inventionwhere at least two unidirectional SAW transducers 600 are placed inparallel acoustic subchannels 610 of aperture W, separated by a distancecomparable to the wavelength in the direction perpendicular to thepropagation direction of the wave, and electrically connected inparallel. In the particular case shown, four unidirectional transducersare connected in parallel. The transducers are separated by narrow busbars 682, 683, having widths on the order of one wavelength. The SAWSgenerated by all transducers towards the forward direction (to theright) create a single acoustic beam with the total aperture close to4W. Parallel connection of said transducers decreases the resistive anddiffractional losses.

Illustrated in FIG. 7 is a SAW device used for SAW tag applicationwherein one unidirectional transducer 700 and SAW identification tags720 are applied in the acoustic channel 710 to create response signalscorresponding to the identification code of the device bearing such sawidentification tags 720.

It is evident that several known solutions used in SAW devices can beapplied to the present invention. For example, said unidirectionaltransducer can be chirped, i.e., it includes a number of said sections,situated quasi-periodically with the period equal to the length of thesection or greater than the length of the section by an integer numberof wavelengths, the wavelength slowly changing along the length of thestructure.

Another possibility is the use of a fan-shaped structure. It is clearfor a person skilled in the art that such standard variants are includedin the scope of this invention.

The use of one-fourth wavelength and wider electrodes in the presentinvention allows the manufacturing of the devices with the standardlithographic techniques up to the frequency range of 2–3 GHZ. The use ofwide and floating electrodes as reflectors dramatically decreases theresistive losses, especially in applications where a wide aperture isimportant, such as SAW tags.

Turning now to FIGS. 8A–8F, illustrated are diffracted wave forms foramplitude 810 and phase 820 of an interrogation pulse on a piezoelectricsubstrate at various distances from a transducer. Dealing with the twoextremes, illustrated in FIG. 8A is a near field waveform and in FIG. 8Fis a far field waveform. As can be seen in FIG. 8F, as the interrogationpulse gets further from the transducer, it spreads and diffuses. FIG. 8Fshows that both the amplitude 810 and phase 820 of the pulse have a mainlobe 830 and sidelobes 835, which appear as shoulders to the main lobe830. These lobes 830, 835 constitute the majority of the energy thatwill be reflected. It is beneficial to try to capture as much as thisreflected energy as possible.

Turning now to FIG. 9, illustrated is a reflector on a piezoelectricconstructed in accordance with the present invention that is arranged tosubstantially match the diffraction field of the interrogation pulse.Illustrated in FIG. 10, is a representative SAW tag 1000 usingreflectors 1010 of the type illustrated in detail in FIG. 9. Asillustrated, a reflector 1010 constructed in accordance with the presentinvention can be used in either the near field 1020 of the interrogationpulse to capture the energy depicted in FIG. 8A, or in the far field1030 to capture the energy depicted in FIG. 8F. Of course, a pluralityof such reflectors 1010 will be generally used in the case of a SAW RFIDtag 1000, which means that a reflector 1010, or reflectors 1010, of thetype described herein may be used in the near field 1020 on a substrateand others in the far field 1030 on the same substrate, all of which iswithin the intended scope of the present invention.

As illustrated in FIG. 9, the reflector will be shaped or curved tosubstantially match the contour of constant phase or contour of constantamplitude, or both, in the diffraction field of the interrogation pulse.Of course, if non-metallic reflectors are used or if the reflectors arenot electrically isolated, in the case of metallic reflectors, it isonly necessary to match the contour of constant phase of theinterrogation pulse.

There can be a substantial advantage to segmented the reflector to formthe fundamentally curvature shape that matches the contour of constantphase or the contour of constant amplitude in the diffraction field ofthe interrogation pulse. It is advantageous from a manufacturingviewpoint since it is more difficult to form curves that match thesignal contours than using small straight lines to approximate it.Another advantage of segmentation, in the case of metallic reflectors,is that the voltage in the reflector can be more easily controlled. Inone embodiment of the invention it has been found to be advantageous tosegment the reflector into segments separated by spaces approximatingone-quarter of the central frequency of the SAW being propagated by thetransducer.

Another advantage of curving the reflector is that it permits thereflected signal to be focused in a manner that allows the responsepulse to have the same approximate shape as the interrogation pulse whenit is detected by the transducer. Thus the response pulse will havesubstantially the same phase and amplitude as the portion of theinterrogation pulse that reaches the reflector.

The design described herein can be used for both open circuit reflectorsor short circuit reflectors. It can also be used for non-metallicreflectors. In the case of non-metallic and short circuit metallicreflectors it is only necessary to match the contour of constant phaseof the interrogation signal.

As can be seen from FIG. 8F, it is advantageous for the reflector toencompass encompasses the main lobe 830 and first sidelobes 835 of theinterrogation pulse. Of course in the case of a near field reflector, ascan be seen from FIG. 8A, it is only necessary for the reflector toencompass the main lobe 830, since the sidelobes 835 are virtuallyundistinguishable. It has been found to be advantageous, in the case ofthe far field when a segmented reflector is used, to use four segmentsfor each of the sidelobes 835 and eight segments for the main lobe 830,for a total of sixteen segments in the reflector.

Turning to FIG. 11, illustrated is a representative example of asegmented metallic open-circuit reflector constructed in accordance withthe present invention that substantially matches both the amplitude anda contour of constant phase of the diffraction field of an interrogationpulse.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A surface acoustic wave (SAW) device, comprising: a defined area on apiezoelectric substrate within which is located an open circuitreflector perpendicular to the SAW direction of propagation; and a pairof low reflectivity transducer electrodes located within said definedarea and connected to opposing bus bars, said electrodes perpendicularto the direction of said SAW propagation and positioned with theexcitation center of said pair of electrodes located about seven-eighthsof a Rayleigh wavelength at a center frequency of said SAW from saidreflector.
 2. The device as recited in claim 1 wherein said defined areais a distance about equal to twice said Rayleigh wavelength at a centerfrequency of said SAW, plus said Rayleigh wavelength of a centerfrequency of said SAW multiplied by a positive integer minus one.
 3. Thedevice as recited in claim 1 wherein said electrodes are separated by adistance of about one-half of said Rayleigh wavelength at a centerfrequency of said SAW.
 4. The device as recited in claim 1 wherein saidelectrodes each have a width of about one-fourth of said Rayleighwavelength at a center frequency of said SAW.
 5. The device as recitedin claim 1 wherein said open circuit reflector is aluminum.
 6. Thedevice as recited in claim 1 wherein said electrodes are aluminum. 7.The device as recited in claim 6 wherein said electrodes have a lowtotal reflectivity.
 8. The device as recited in claim 6 wherein saidelectrodes have a thickness about equal to one-tenth of said Rayleighwavelength at a center frequency of said SAW.
 9. The device as recitedin claim 1 further comprising utilizing a piezoelectric substrate suchthat the mechanical reflectivity has an opposite sign as compared to theelectrical part of the reflectivity of said pair of electrodes and saidreflector.
 10. The device as recited in claim 1 wherein saidpiezoelectric substrate is 128° LiNbO3.
 11. The device as recited inclaim 1 wherein the width of said electrodes is less than one-quarterwavelength.
 12. The device as recited in claim 1 wherein the width ofsaid reflector is between 0.3 wavelength and 0.5 wavelength.
 13. Thedevice as recited in claim 1 further comprising at least two opencircuit reflectors having a width of about one-fourth wavelength andseparated by a distance of about one-half wavelength.
 14. The device asrecited in claim 1 wherein said excitation center of said pair ofelectrodes located about seven-eighths of a Rayleigh wavelength at acenter frequency of said SAW from said reflector is varied by an amountequal to plus or minus ten percent of said seven-eighths of a Rayleighwavelength.
 15. The device as recited in claim 1 further comprising atplurality of pairs of electrodes, each of said pairs of electrode offsetat a distance equal to a number of wavelengths and connected to the samebus bars in the same sequence of polarity.
 16. The device as recited inclaim 1 further comprising a plurality of equivalent reflectors, witheach reflector offset at a distance equal to a number of wavelengthsfrom each other such that said reflectors do not overlap saidelectrodes.
 17. The device as recited in claim 1 further comprising aperiodic set of said defined areas.
 18. The device as recited in claim17 wherein said periodic set of said defined areas are situatedquasi-periodically with the period equal to the length of said definedarea or greater than the said length by an integer number ofwavelengths, said wavelength slowly changing (chirped) along the lengthof said piezoelectric substrate.
 19. The device as recited in claim 1wherein said device is used as a unidirectional SAW transducer forapplication with low loss.
 20. The device as recited in claim 1 furthercomprising a plurality of said defined areas are placed in parallelacoustic sub channels, separated by a distance comparable to thewavelength in the direction perpendicular to the propagation directionof the wave, and electrically connected in parallel.
 21. The device asrecited in claim 1 wherein said defined space is located on a SAWidentification tag.